1. Field of the Invention
The invention described herein relates generally to monopulse antennas and more particularly to parabolic reflector feed antennas for the generation of monopulse beams.
2. Description of the Prior Art
A parabolic reflector illuminated symmetrically by a primary source at the focal point will produce a symmetrical far field pattern centered along the antenna axis through the apex and focal point, referred to generally as the boresight axis. When the primary source illuminating the parabolic reflector is laterally displaced in the focal plane from the focal point F by a small distance .DELTA..sub.X, the phase center will remain close to the original position but the amplitude pattern will be squinted off the boresight axis at an angle of approximately .DELTA..sub.X/F. A pair of feeds spaced symmetrically from the focal point will then produce symmetrically overlapping amplitude patterns in the plane determined by the axis through the feeds offset points and boresight axis. This characteristic of parabolic reflectors is utilized in amplitude sensing monopulse systems wherein tracking information is obtained by means of fixed beams squinted off the boresight axis to establish a cross over level. In the simplest form, a two axis monopulse system employs four feeds in the focal plane of the parabolic reflector and a comparator network coupled to these feeds to obtain sum and difference beams necessary for monopulse operation. Comparison of the sum and difference patterns is then used to generate an error voltage representing the angle of arrival of the received signals.
The principle parameters of interest in a monopulse system are the gain of the sum beam, which effects the maximum range of the system and the difference pattern slope, which effects the off boresight angle sensitivity. In a four horn feed system these parameters are not independent, the sum pattern gain decreases while and the difference pattern slope increases with increasing squint angle. Generally a three to four db cross over between the patterns of adjacent feeds is a compromise selection.
Improved sum pattern gain and difference pattern slope may be achieved with a twelve element feed system suitably coupled through hybrid couplers to the sum and two difference channels of the monopulse antenna. This twelve element feed is configured with twin rows, having four elements each, along orthogonal axes in the focal plane to form a cross of elements centered about the focal point of the parabolic reflector. The four central elements of the cross are common to each axis and are used to generate the sum pattern. Two groups of four feeds about each axis are used to generate the two difference patterns. This configuration provides a greater sum pattern gain for a given difference pattern slope and a greater product of sum pattern gain and difference pattern slope than that provided by the four horn feed.
Another configuration that exhibits improved monopulse performance over the four element feed is a multihorn multimode feed system. In this system, four sectional horns of equal flare angle, are symmetrically positioned with the flare apex parallel about the focal point of a parabolic reflector. A generator of natural modes is coupled to the throat of each horn. This mode generator is coupled through a hybrid junction network to a sum channel port and two difference channel ports. Sum channel excitations produce even modes in each of the two center horns thereby providing the illumination of the parabolic reflector for the sum beam. One difference port is coupled to the mode generators to produce even mode exitations into adjacent horns and even mode exitations in a second pair of adjacent horns that have a polarity opposite that of the excitations in the first pair, thus illuminating the parabolic reflector in a manner to form a difference beam for one axis of the system. The second difference channel is coupled to the mode generators to establish odd modes and the two central horns, thereby providing an illumination function that establishes a difference beam about the second axis of the system. This multihorn, multimode feed is equivalent to an eight horn single mode system and requires the same number of hybrids to form the beams.
In the four, eight, and twelve horn systems the number of hybrids required is equal to the number of horns in the system. These configurations are complex, bulky, and do not provide independent control or isolation of the sum and difference channels.
Independent control of the sum and difference patterns and interchannel isolation may be achieved with five horn feed systems. A central horn with its phase center positioned at the focal point of the parabolic reflector with four horns equally spaced about its perimeter is independently fed to provide the illumination function for the sum pattern. The outer horns may be coupled to hybrid junctions in pairs or coupled to a hybrid junction network, depending upon the positioning of the outer horns, to provide the required monopulse difference channels. Complete isolation is achieved between all channels when the outer horns are coupled in pairs, while a gain increase for the difference channels is realized over that of the isolated channels when the four outer horns are coupled through a hybrid junction network to form the difference beam patterns. Though this system provides an isolation between the sum and difference patterns, the size of the central horn limits the cross over level of the squinted beams that are combined to form the difference pattern, thereby limiting the gain and error slopes achievable for these patterns.